What’s The Impact of Planting Date on Corn Yields in Ontario?

Simple Summary

  • Ontario data suggests planting date from late April to mid-May has minimal impact on corn yields
  • After mid-May, yield declines can start and are quickest in shorter season areas
  • Balance any risks of pushing planting dates (e.g. unfavourable weather events or unfit soil) to the yield losses expected for delaying planting within the confines of your operation, soil and weather forecast
  • Uniform and unimpeded emergence, development and growth are also a foundation for high yielding corn crops

A common refrain at the ag breakfast meetings held across the province every spring is “we often farm on last year’s weather”. Some of these comments float around on planting dates and yield potential.

For example, 2020 and 2021 benefitted from very early springs. While not always warm, dry sunny conditions allowed field preparation to start as early as late March in some cases and planting was well underway by May 1. Soil conditions generally remained excellent through both springs and large areas of the province experienced exceptional yields.

If early springs are our reference point, years where little field activity has started by May 1 might seem late. So, this begs the question – are we giving up yield potential every day we are waiting on field conditions to shape up now that the calendar has switched to May?

What’s the Impact of Planting Date on Corn Yields?

The most complete Ontario dataset investigating planting date impacts on corn yields was conducted by Dr. Dave Hooker and Greg Stewart from 2006-2009. They looked at yields from 8 adapted hybrids at 3 locations (Elora, Exeter, Ridgetown) for three planting dates:

  • late April/early May (~April 20 to May 5)
  • mid-May (~May 10- May 20)
  • late May/early June (~May 25 – June 5).

Figure 1. Planting date vs. relative grain corn yield at three locations in Ontario, 2006-2009.

While timely planting is important, this data does not support concerns about daily declines in corn yields from later April through mid-May. On average, there was very little yield difference between these two planting periods, suggesting there may be a reasonable planting window in Ontario where yield losses are minimal when planting an adapted hybrid.

Beyond mid-May, yield reductions can start, but rate of decline depends on where you are. In this dataset, yield losses increase the fastest at Elora followed by Exeter then Ridgetown (Figure 1 and Table 1). The differences might be mostly related to reductions in growing season heat units from Elora to Ridgetown, as other work has shown (Doerge et al, 2021). Soils at these trials also generally become heavier moving along the same gradient, so perhaps there is also increasing yield risks from unfit planting conditions for very early planting at Exeter and Ridgetown.

Table 1. Yield expectation for various planting dates at Elora, Exeter and Ridgetown Ontario from 2006-2009 planting date trials.

May 10 or EarlierMay 15May 20May 25May 30Jun 5
------------------------------------------------------------------ Yield Expectation (% of maximum yield) ------------------------------------------------------------------

Pros and Cons of Pushing Planting Dates

Decisions are never black and white. Beyond just yield potential, there may be many pros and cons when deciding to push planting dates:


  • ensure entire corn acreage has been planted before yield losses can start to accrue with planting dates past mid-May
  • reduce drying costs, especially important with high energy prices
  • reduce maturity and quality/yield risks in shorter season areas
  • increase likelihood of a timelier harvest
  • potentially capture extra yield by extending maturity of early planted corn
    • in years with above normal CHU accumulation, there can be strong yield responses for longer CHU hybrids
    • this is only a pro for early planting if a meaningful amount of extra heat units are actually received prior to May… not realized in years like 2022 where temperatures remain cool through April


  • vigour risks when planting ahead of unfavourable weather events
    • it is very hard to predict outcomes with any accuracy, but traditional wisdom to avoid potential risks is to stop planting 24-48 hours prior to weather events bringing cold rain or snow
    • if you decide to push the risk, experiences in recent years suggests at least leaving hybrids known to be weak for early season stress until after such weather events
  • planting into unfit soil
    • there can be yield risks if planting conditions are compromised in efforts to plant early (compaction, unruly seedbeds, planter opener/trench smearing, poor slot closure etc.) where the crop is not able to achieve uniform, unimpeded emergence, development, and growth
    • prior to the middle of May, limited yield rewards are expected for pushing planting dates of adapted hybrids
    • consider the risk of yield loss from planting into unfit soil conditions relative to the yield risk of waiting for fitness to improve, within the confines of your operation (number of days required to plant corn acreage), soil (unfit soil yield risks, fitness window) and weather forecast (expected workable days before yield loss becomes an issue)

Examples of How This Data Might Help

Some examples for how planting delay yield risk data might be useful when weighing planting decisions:

  • It’s May 1, your neighbour is planting corn but you are unsure about soil conditions. You feel you have experienced issues pushing conditions on your soils in the past, so this could present real yield risk. Waiting 2 more days for soil conditions to improve when the forecast looks favourable for completing planting on time (e.g. before planting delay yield losses become an issue in your area) likely presents no yield risk from a delayed-planting perspective.
  • Fast forward to the second half of May, and depending where you are in the province, each day of delay might mean a 0.5-1% reduction in yield potential. Balance expected soil condition yield risks to these reductions for waiting.
  • It’s May 15 and you are in a shorter season area. Given the forecast, corn not planted in the next 2 days likely means a 7-10 day delay. This delay likely represents a 7% or more yield reduction, among other costs/risks.  

Corn Replant Decision Aid

If you would like to test your own planting date yield potential scenarios based on this dataset, you can do so with the Ontario Corn Replant Decision Aid tool available at the homepage of gocorn.net.

Figure 2. Corn Replant Decision Aid tool available at gocorn.net

Looking back to the excellent yields of 2020 and 2021, not only was planting early, but soil conditions also generally remained excellent throughout the spring. While timely planting is important, uniform, unimpeded emergence and development throughout the growing season are also a foundation for high yielding corn crops.


Doerge, T., M. Jeschke and P. Carter. 2021. Planting outcome effects on corn yield. Pioneer. https://www.pioneer.com/us/agronomy/corn_planting_outcome_effects.html (accessed 28 Apr. 2022)